1. Field of the Invention
The present invention relates to a process for the production of ammonium thiosulfate and, more particularly, a process for producing ammonium thiosulfate from a feed gas stream containing a mixture of ammonia and hydrogen sulfide.
2. Description of the Prior Art
Ammonia and hydrogen sulfide gases are frequently found together in mixtures with water or other gaseous components. Such mixtures are often a by-product of petroleum refining and chemical process, particularly where crude oils and feedstocks containing nitrogen and sulfur compounds are processed. Not only can ammonia and hydrogen sulfide occur naturally in the raw material, they can also be produced as decomposition products from such processes as distillation, cracking, and coking. The nitrogen and sulfur content of a feed material can be reduced by conversion to ammonia and hydrogen sulfide in catalytic hydrogen treating processes such as hydrodesulfurization, hydrocracking, and reforming. Mixtures of ammonia and hydrogen sulfide can also result from processes such as ore reduction, metal refining, papermaking, and coal distillation.
These by-product gases were once considered waste and either incinerated or burned in combustion furnaces to recover their fuel value. Even so, combustion is not a desirable means of disposal, as the oxides of nitrogen and sulfur produced and found in the flue gases are corrosive, cause unsightly stack plumes, and contribute to atmospheric pollution.
Ammonia and hydrogen sulfide are also found in sulfidic waters produced from such processes or are obtained by scrubbing the aforementioned gases to remove the ammonium and hydrogen sulfide therefrom. In the past, waste sulfidic waters were frequently disposed of by discharging them to streams, rivers, lakes, oceans, or other convenient bodies of water.
Out of growing concern for water and air pollution, coupled with stringent regulations regarding plant water and gaseous effluent quality, various processes have been developed to treat these by-product effluent streams. Stripping of the noxious ammonia and hydrogen sulfide from the sulfidic waters has been used to improve the quality of effluent waters; however, the stripped gases, commonly referred to as sour water stripper off gas (SWSG) still present a disposal problem. Most prior art processes that have dealt with the SWSG stream have either been complicated, required extensive plant investment, entailed high operating costs, failed to produce a readily marketable product for which a reasonably stable demand existed, or were unsuitable for the treatment of relatively small or intermittent by-product streams. Although some of these processes provide a suitable means of disposing of the by-product effluents, they fail to yield products of commercial value.
It clearly would be desirable to have a method for processing a gas stream containing ammonia and hydrogen sulfide whereby a salable product could be produced. To this end, U.S. Pat. No. 3,431,070 discloses a process for treating ammonia and hydrogen sulfide gas mixtures to produce ammonium thiosulfate and sulfur, the sulfur typically being present in the aqueous ammonium thiosulfate solution as finely divided crystals.
It is therefore an object of the present invention to provide a process for recovering the value of the ammonia present in an SWSG by converting it to ammonium thiosulfate, a readily marketable chemical.
Another object of the present invention is to provide a process for producing ammonium thiosulfate from a gas mixture comprising ammonia and hydrogen sulfide by utilizing the ammonia therein without the necessity of separating it from the other components of the mixture and without the need for ammonia from any additional source.
Still a further object of the present invention is to provide a process for the production of ammonium thiosulfate from a gas mixture comprising ammonia and hydrogen sulfide wherein hydrogen sulfide in excess of stoichiometric requirements is selectively rejected as an off-gas stream essentially free of ammonia and sulfur dioxide.
Yet another object of the present invention is to provide a process for producing ammonium thiosulfate from a gas mixture comprising ammonium and hydrogen sulfide wherein an effluent stream from the process, whether gaseous or liquid, does not adversely affect the environment or subsequent downstream processes.
According to the process of the present invention, a feed gas mixture comprising hydrogen sulfide and ammonia is contacted, preferably in a spray-type absorber, with an aqueous absorbing stream comprising ammonium thiosulfate, ammonium bisulfate, and ammonium sulfite in a first reaction zone. The contacting is conducted under conditions that limit the conversion of sulfite to thiosulfate and produces an ammonia-rich absorbing stream that has a lower concentration of sulfitexe2x80x94i.e., a sulfite-lean stream. Unreacted hydrogen sulfide is rejected from the ammonia-rich absorbing stream in the first reaction zone, producing or leaving a liquid, first reaction zone product free of unabsorbed gases. Sulfur dioxide gas from a suitable sulfur dioxide-containing gas stream is absorbed in the ammonia-rich absorbing stream in the absence of any substantial quantity of hydrogen sulfide in a second reaction zone to produce a second reaction zone product free of unabsorbed gases. At least a portion of the second reaction zone product is recycled to the first reaction zone. An aqueous product stream of ammonium thiosulfate is recovered from one of the first or second reaction zone products.
In the process of the present invention, by limiting the conversion of sulfite to thiosulfite, there is produced a stream with a lower concentration of sulfite, the unreacted hydrogen sulfide being rejected from the sulfite-lean stream in the first reaction zone. The sulfite-lean stream from the first reaction zone is passed to a second reaction zone wherein it contacts a gaseous stream containing SO2 that is absorbed from the gaseous stream, converting sulfite ion to bisulfite.